Various configurations of cartridge heaters are known in the prior art. A typical cartridge heater comprises a resistance wire heating element coiled around an elongate core of insulating material, and an elongate metal sheath that is coaxial with the coiled heating element and core and radially spaced from the coil. An insulating material having an optimum combination of relatively high thermal conductivity and relatively low electrical conductivity is used to fill the space between the coil and the sheath. Granulated magnesium oxide is one substance known to be suitable for the purposes of serving as the insulating filler material. Toward the end of the manufacturing process, the granulated magnesium oxide is introduced into the sheath, for example by gravity feed. Upon sealing the sheath, the sheath is subjected to compression forces, for example, by swaging, thereby compacting the granulated magnesium oxide to improve its dielectric and thermal conductive properties.
Depending upon the intended application, cartridge heaters of varying sizes and voltage ratings may be required. In many applications, a relatively high voltage, on the order of 480 volts or so, may be desired.
With current manufacturing technology, it has proven to be a challenge to reliably produce high-voltage cartridge heaters whose sheaths have an outside diameter of one-half inch or less. Typically, such a combination of factors (small diameter, high voltage) tends to lead to problems with dieletric breakdown and current leakage problems. In some cases, operating parameters such as dielectric strength and current leakage must be kept within predetermined limits in order for the cartridge to meet certain industry standards, such as those standards established by Underwriters' Laboratories (for example, the UL 499 and UL 544 standards).
One apparent reason for such problems is that for smaller cartridges, tight manufacturing tolerances cannot be repeatably maintained with current cartridge filling equipment and manufacturing procedures. An element core which is as little as 0.001 inch off-center after filling can compromise the dielectric strength of the element. Thickness, compaction, and therefore density of the filler material may be inadequate for reliable dielectric strength at the operating temperatures of the heater.
As a result of these considerations, high-voltage cartridge heaters are traditionally only offered in diameters larger than one-half inch, or employ designs which do not qualify for certification under the applicable industry standards.